In an inkjet printer, ink droplets held in an ink reservoir are ejected from a nozzle by pressing the ink reservoir provided in an inkjet head. The droplet ejected from the nozzle flies while tailing with the head and the tail thereof each having different flying velocity and time. This tailing sometimes causes formation of undesirable small droplets called satellites associated with the preceding main droplet. A non-aqueous ink in particular tends to form such satellites due to a low dynamic surface tension thereof. Further, a high-speed printer is prone to form satellites due to its high drive frequency, sometimes causing significant blots in printed images.
As a method to prevent such satellites from occurring, Japanese Patent Application Laid-Open No. 2007-326930 discloses a non-aqueous inkjet ink containing a polyvinylchloride resin having sulfonic acid groups or sulfuric acid groups. As a reason for the ink's preventing satellites, it is speculated that the sulfonic or sulfuric acid groups adsorb on dispersed pigments to prevent their agglomeration by increasing steric hindrance of the pigments, and thus improve storage stability as well as preventing satellite from occurring even when the ink is ejected continuously.
Meanwhile, the incorporation of a resin dispersed in a solvent is practiced to prevent pigments from agglomerate. As the resin, Japanese Patent Application Laid-Open No. 2005-171032 discloses resin particles comprising a core of C1-4 alkylester of poly(meth)acrylic acid and a shell of C4-10 alkyl ester of poly(meth)acrylic acid, Japanese Patent Application Laid-Open No. 2007-197500 discloses particles of copolymer, which also function as pigment dispersants, of C12-25 alkyl ester of poly(meth)acrylic acid and acrylic monomer having a specific functional group such as glycidyl group, and Japanese Patent Application Laid-Open No. 2010-1452 discloses a copolymer resin, which is improved in pigment dispersion capability by incorporating a urethane moiety in C12-25 alkyl ester of poly(meth)acrylic acid.